A peitition couldl be filed with the court for a modification of the visitation schedule, if for example the person supervising the child during father's work hours is inapproriate or there is other cause. You do not say how old the child is, whether the child is left alone, etc. - all factors that would enter into the judge's decision. One can not however, simply act on their own and take away court ordered visitation.
Only a court upon a filing of the proper action and with proper evidence can deal with this issue.
If the mother does, it is actionable as a contempt. However, if the visitation schedule is bad for the father, the father should change it ASAP
Child support payments and visitation are independent of each other. The mother cannot unilaterally change the visitation schedule. If the visitation schedule is not allowing you to see your child, you should look into filing a complaint for modification to have it changed. Consider contacting an attorney to discuss the specifics, or the court may have an attorney of the day program to offer some guidance.
DISCLAIMER: This answer is provided in response to a "hypothetical" question and provided for general, informational purposes and does not create an attorney/client relationship. The information presented is not legal advice and may change based additional information and research. It is recommended that you speak to an attorney to discuss your specific legal issues.
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